The invention relates generally to a current limiter and more specifically to a superconducting fault current limiter.
Current limiting devices are critical in electric power transmission and distribution systems. For various reasons such as lightening strikes, short circuit conditions can develop in various sections of a power grid causing sharp surge in current. If this surge of current, which is often referred to as fault current, exceeds the protective capabilities of the switchgear equipment deployed throughout the grid system, it could cause catastrophic damage to the grid equipment and customer loads that are connected to the system.
Superconductors, especially high-temperature superconducting (HTS) materials, are well suited for use in a current limiting device because of their intrinsic properties that can be manipulated to achieve the effect of “variable impedance” under certain operating conditions. A superconductor, when operated within a certain temperature and external magnetic field range (i.e., the “critical temperature” (Tc) and “critical magnetic field” (Hc) range), exhibits no electrical resistance if the current flowing through it is below a certain threshold (i.e., the “critical current level” (Jc)), and is therefore said to be in a “superconducting state.” However, if the current exceeds this critical current level the superconductor will undergo a transition from its superconducting state to a “normal resistive state.” This transition of a superconductor from its superconducting state to a normal resistive state is termed “quenching.” Quenching can occur if any one or any combination of the three factors, namely the operating temperature, external magnetic field or current level, exceeds their corresponding critical level. Mechanisms, using any one or any combination of these three factors, to induce and/or force a superconductor to quench is usually referred to as a trigger mechanism.
A superconductor, once quenched, can be brought back to its superconducting state by changing the operating environment to within the boundary of its critical current, critical temperature and critical magnetic field range, provided that no thermal or structural damage was done during the quenching of the superconductor. HTS material can operate near the liquid nitrogen temperature 77 degrees Kelvin (77K) as compared with low-temperature superconducting (LTS) material that operates near liquid helium temperature (4K). Manipulating properties of HTS material is much easier because of its higher and broader operating temperature range.
For some HTS materials, such as BSCCO, YBCO, or MgB2 elements, there often exists within the volume of the superconductor, non-uniform regions resulting from the manufacturing process. Such non-uniform regions can develop into the so-called “hot spots” during the surge of current that exceeds the critical current level of the superconductor. Essentially, at the initial stage of quenching by the current, some regions of the superconductor volume become resistive before others do due to non-uniformity. A resistive region will generate heat from its associated i2r loss. If the heat generated could not be propagated to its surrounding regions and environment quickly enough, the localized heating will damage the superconductor and could lead to the breakdown (burn-out) of the entire superconductor element.
A magnetic field is used to trigger HTS materials to improve speed and uniform quenching during transition from superconducting to normal resistive state. In some Superconducting Fault Current Limiter (SCFCL) designs, external windings (coils) are used to generate the trigger magnetic field.
US Publication US2005/0099253A1, published on May 12, 2005, discloses a superconducting current limiting device comprising a superconductor body electrically connected in parallel with a shunt coil wherein the shunt coil is in tight contact with the external surface of the superconducting body. The shunt coil has an external shape to allow a circular current to flow. This publication does not disclose or teach the elimination of the external shunt coil to use other means for generating a magnetic field to assist in quenching.
U.S. Pat. No. 6,043,731, issued on Mar. 28, 2000, discloses a current limiting device having a superconductor, a shunt coil wrapped around the superconductor and connected in parallel with the superconductor, wherein the shunt coil generates a magnetic field to assist in quenching the superconductor. The shunt coil is controlled by active means. This patent does not disclose or teach the elimination of the external shunt coil to use other means for generating a magnetic field to assist in quenching.
As the need for higher power and higher voltage applications of fault current limiters increases, designing a device with less complexity and still using magnetic field for triggering becomes a challenge. Optimizing the fault current limiter design with fewer magnetic field generating components, or even better, fewer overall components, is important to design a reliable high voltage device at a transmission system level.
Since the discovery of the high temperature superconductors, various forms and types of SCFCL designs, such as inductive and resistive fault current limiters, have been reported. There are also various types of trigger mechanisms used in order to improve the speed and uniformity of the quenching process of the superconducting materials. In most such techniques, external means such as external magnetic field, coupling magnetic circuits or transformers, and active switching circuits based on power electronics, have been used. U.S. Pat. No. 6,137,388, entitled, Resistive Superconducting Current Limiter and U.S. Pat. No. 6,664,875, entitled, Matrix-Type Superconducting Fault Current Limiter, use external magnetic circuits to generate the triggering magnetic field. Scaling these designs to higher voltage and higher power applications remains a challenge, especially at the 138 kV or higher transmission voltage levels. In most cases the high voltage design is considered the most challenging aspect of the transmission system SCFCL development.
As the power and voltage requirement increases, the number of components (superconductors and magnetic field coils) increase, which adds to the complexity of the device. Reducing the number of parts is one of the ways to improve reliability of the device.
For the reasons described above there is therefore a need for a simplified design and to improve reliability of the SCFCL device for transmission system applications in higher voltage applications.